update the dependency on Twisted to >= 10.1

[davidsarah]

There are three reasons to update the Twisted requirement to >= 10.1:

• it would fix our indirect dependency on pywin32 on Windows when using Twisted < 9.0 (#1274)
• it would ensure that the twisted.internet.inotify package is available for use by the drop-upload feature on Linux (#1429)
• it would make the FTP server work without users potentially having to patch Twisted (see ​http://twistedmatrix.com/trac/ticket/3462 and docs/frontends/FTP-and-SFTP.rst@5036#dependencies).

I'm splitting this into its own ticket so that it can be reviewed and discussed separately (I'd previously bundled the update into the patch to fix #1435).

[davidsarah]

Update the dependency on Twisted to >= 10.1. This allows us to simplify some documentation: it's no longer necessary to install pywin32 on Windows, or apply a patch to Twisted in order to use the FTP frontend. fixes #1274, #1438. refs #1429

[davidsarah]

This includes the previous patch but also removes the check in ftpd.py that Twisted supports async close, since that check is no longer needed. (Opinions wanted on whether it is desirable to remove this check.)

[zooko]

I reviewed attachment:update-twisted-dependency-2.darcs.patch​ and it is good.

[zooko]

Oh, and yes I think we should remove the specific check.

[david-sarah@…]

In 8b4082677477daf1:

Update the dependency on Twisted to >= 10.1. This allows us to simplify some documentation: it's no longer necessary to install pywin32 on Windows, or apply a patch to Twisted in order to use the FTP frontend. fixes #1274, #1438. refs #1429